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SubjectRe: Fwd: Re: [PATCH] USB: add USB test and measurement class driver
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On Thursday 28 August 2008 18:21:09 Greg KH wrote:
> On Thu, Aug 28, 2008 at 11:37:24AM +0200, Marcel Janssen wrote:
> > Hello Greg,
> >
> > I've got me a bit confused there :-)
> >
> > We currently only use the suspend/resume in our device which works on a
> > received request from the PC.
> > Basically sending suspend will make the device enter this mode. When the
> > PC enters this mode, it's also clear it will send a resume when resuming
> > from standby. But, how is this supposed to work when the PC is not in
> > sleep mode.....how will the device wake up ?
>
> It should "automatically" wake up if something happens to it. The USB
> spec calls this out as to how this should work. If the devices don't
> support this, we don't have to add support for it :)

Actually this is a good point.

The USB spec mentions that the device should suspend when a constant idle
state is detected for more than 3.0ms.

I either have not seen that happening before or I missed something when
writing my firmware. I'll keep an eye on my USB analyzer though :-)
Although it's a good thing to keep to the spec, I also believe that in the
case of a test and measurement device suspending the device may be an
unwanted behaviour but perhaps I do misinterpret this part of the spec here.

Anyway, this is no issue for self-powered devices, which is probably the
majority in this class because most test & measurement devices need a good
power supply and USB just isn't fit for that unless you're able to make a
clean power from that (which is possible of course).
Our device is both USB and self powered, but when in USB powered mode it will
only suspend when the host goes in sleep mode.

To be honest I would put suspend mode far down on the priority list :-)

regards,
Marcel




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